How can mesh have different material than particles?
For some reason, I can't open your belnd file but not to worry, I'll show you anyway.

Create two materials (Ground and Grass for example), then go into edit mode and assign your ground material to your ground plane.

Next go into particle settings and scroll into render settings and apply the grass material. this will apply the other material to your particles. In this order it should work fine.

See image below:


Hope that helps!